Acoustic termination for a pressure monitoring system

ABSTRACT

Aspects of the invention relate to an improved acoustic termination for a pressure monitoring system. The termination can include an inner tube having a passage extending therethrough. One end of the inner tube can extend into an open end of an outer tube, which is closed at its opposite end. The portion of the inner tube that extends into the outer tube can include a plurality of apertures. Sound absorption material can fill the space between the outer tube and the portion of the inner tube extending therein. A pressure signal entering the acoustic termination can be substantially dissipated and can minimize signal reflection from the termination. As a result a more accurate pressure measurement can be obtained by a pressure measurement device that operatively connects to the passage upstream of the termination.

FIELD OF THE INVENTION

The invention relates in general to pressure monitoring systems and, more particularly, to pressure monitoring systems for turbine engines.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, a turbine engine 10 includes a compressor section 12, a combustor section 14 and a turbine section 16. During the operation of the turbine engine 10, fuel and air can be mixed, ignited and burned in the combustor section 14, producing superheated gases at high pressures. Dynamic pressure waves, sometimes referred to as combustion dynamics, can develop during the combustion process. If sufficiently high in amplitude, these dynamic pressure waves can cause mechanical damage and reduce engine life. Accordingly, pressures in the combustor section 14 are monitored during engine operation.

Normally, a pressure transducer cannot be directly attached to the combustor section 14 because the pressure transducer cannot withstand the temperatures in the combustion chamber. However, FIG. 1 shows one known pressure monitoring system 18 that is configured to minimize such concerns. A sampling tube 20 is connected to and extends away from the combustor section 14 to an acoustic termination 22. As shown in FIG. 2, the termination 22 is formed by a plurality of smaller tubes 24 enclosed within the larger sampling tube 20. A pressure transducer can be connected to the sampling tube 20 upstream of the termination 22.

Thus, a pressure signal-or wave from the combustor section 14 enters the sampling tube 20 and travels along the sampling tube 20 such that the hot gas from the combustor section 14 cools to a temperature suitable for interaction with the pressure transducer 28. The pressure signal engages the pressure transducer 28, which uses the signal to measure the pressure in the combustor section 14. The pressure signal can continue through the sampling tube 20 and encounter the termination 22, which can dampen the signal primarily through viscous dissipation. The termination 22 can minimize reflection of the pressure wave to prevent contamination of the true pressure signal that engages the pressure transducer 28.

While such a system 18 has proven adequate for measuring pressure, the termination 22 is expensive. Moreover, there are concerns of a decrease in system performance due to signal distortion and contamination. For instance, it has been shown that the resonant frequency of the pressure sampling tube 20 is contained in and repeated in the final pressure measurement. This repetition of the resonant frequency occurs because the pressure signal is not totally dissipated by the termination 22. Acoustically, the termination 22 is not completely anechoic, so a portion of the pressure signal that encounters the termination 22 is reflected back into the pressure sampling tube 20. Consequently, the pressure measured by the transducer 28 is the summation of at least the real pressure signal and the reflected pressure signal, thereby increasing the possibility of signal contamination.

Thus, there is a need for a pressure monitoring system that can improve performance by minimizing signal contamination and distortion. Ideally, such a system can be provided at a reduced cost.

SUMMARY OF THE INVENTION

Aspects of the invention are directed to an acoustic termination for a pressure monitoring system. The system includes an outer tube that has an at least partially open first end and a closed second end. The outer tube has an interior. At least a portion of the interior is filled with sound absorption material, which can be, for example, fiberglass.

The system includes an inner tube with an open inlet end and an open termination end. The inner tube also has an outer peripheral surface and an inner peripheral surface. The inner peripheral surface of the inner tube defines a passage extending through the inner tube. The inner tube operatively engages the first end of the outer tube such that the passage is in fluid communication with the interior of the outer tube. In one embodiment, the first end of the outer tube can provide an opening sized to allow passage of the inner tube such that the opening substantially engages the outer peripheral surface of the inner tube. The inlet end of the inner tube can be connected to a pressurized chamber, such as the combustor of a gas turbine engine. A pressure signal transmitted through the inner tube can be received within the interior of the outer tube and can be dampened by engagement with the sound absorption material.

In one embodiment, the termination end of the inner tube can be positioned substantially proximate the first end of the outer tube. A chamber can be defined within the sound absorption material such that the passage of the inner tube is in fluid communication with the chamber. As a result, a pressure signal exiting the inner tube can be received within the chamber. In another embodiment, a portion of the inner tube including the termination end can extend into the first end of the outer tube. The inner tube can include a plurality of apertures only in the portion of the inner tube that extends into the outer tube. In such case, the sound absorption material can be provided in the space between the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube as well as in the space between the termination end of the inner tube and the second end of the outer tube.

The system can further include a pressure measurement device that is operatively connected in branched relation to the passage. The pressure measurement device can be a pressure transducer. Thus, the pressure measurement device can determine pressure when it is engaged by a pressure signal in the passage.

In another respect, aspects of the invention are directed to a pressure monitoring system. The system includes a pressurized chamber containing at least one pressure signal. The pressurized chamber can be the combustor of a gas turbine engine.

The system includes an outer tube and an inner tube. The outer tube has an at least partially open first end and a closed second end. The outer tube also has an interior. At least a portion of the interior is filled with sound absorption material, which can be, for instance, fiberglass. The inner tube has an open inlet end and an open termination end. The inner tube also has an outer peripheral surface and an inner peripheral surface. The inner peripheral surface defines a passage extending through the inner tube. The inner tube operatively engages the first end of the outer tube so as to be in fluid communication with the interior of the outer tube. Thus, a pressure signal transmitted through the inner tube can be received within the interior of the outer tube and dampened by engagement with the sound absorption material.

In one embodiment, the termination end of the inner tube can be positioned substantially proximate the second end of the outer tube. A chamber can be defined within the sound absorption material such that the passage of the inner tube is in fluid communication with the chamber. As a result, a pressure signal exiting the inner tube can be received within the chamber. In another embodiment, a portion of the inner tube including the termination end can extend into the first end of the outer tube. The inner tube can include a plurality of apertures only in the portion of the inner tube that extends into the outer tube. The sound absorption material can be provided in the space between the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube and the termination end of the inner tube and the second end of the outer tube.

The system can further include a pressure measurement device operatively connected in branched relation to the passage. The pressure measurement device can be a pressure transducer. The pressure measurement device can determine pressure when it is engaged by a pressure signal in the passage.

Another pressure monitoring system according to aspects of the invention includes an outer tube with a first end that is at least partially open and a second end that is closed. The outer tube has an inner peripheral surface.

The system further includes an inner tube with an open inlet end and an open termination end. The inlet end of the inner tube can be connected to the combustor of a gas turbine engine. The inner tube further has an outer peripheral surface and an inner peripheral surface. The inner peripheral surface defines a passage extending through the inner tube. A portion of the inner tube, which includes the termination end, extends into the first end of the outer tube. The inner tube includes a plurality of apertures only in the portion of the inner tube that extends into the outer tube.

Sound absorption material provided in the space between the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube as well as in the space between the termination end of the inner tube and the second end of the outer tube. The sound absorption material can be fiberglass.

The system includes a pressure measurement device operatively connected in branched relation to the passage. The pressure measurement device can be a pressure transducer. Thus, a pressure signal transmitted through the inner tube can engage the pressure measurement device so that the device determines pressure. Further, the pressure signal can be substantially dampened by engagement with the sound absorption material after exiting the inner tube through the apertures and the termination end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly diagrammatic view of a prior system for monitoring pressure in the combustor section of a turbine engine.

FIG. 2 is a cross-sectional view of the prior pressure monitoring system, viewed along line 2-2 in FIG. 1, and showing the acoustic termination of the system.

FIG. 3 is a partly diagrammatic view of a system for monitoring the pressure in the combustor section of a turbine engine according to aspects of the invention.

FIG. 4A is a cross-sectional view of the pressure monitoring system of FIG. 3, viewed along line 4-4, and showing an acoustic termination according to aspects of the invention.

FIG. 4B is a cross-sectional view of the pressure monitoring system of FIG. 3, viewed along line 4-4, and showing an acoustic termination according to aspects of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention provide a pressure monitoring system that can minimize signal contamination and distortion. Embodiments of the invention will be explained in the context of one possible pressure measurement system, but the detailed description is intended only as exemplary. Embodiments of the invention are shown in FIGS. 3-4, but the present invention is not limited to the illustrated structure or application.

A pressure monitoring system 30 according to aspects of the invention can include a number of components. The system 30 can include an elongated inner tube 32. The inner tube 32 can include an outer peripheral surface 34 and an inner peripheral surface 36. The inner peripheral surface 36 can define a passage 38 extending through the inner tube 32. The inner tube 32 can be formed by a single tube or a plurality of tube segments. The inner tube 32 can also include various fittings and/or connectors.

The inner tube 32 and/or the passage 38 extending therethrough can be substantially straight, or the inner tube 32 and/or the passage 38 can include one or more bends, turns, tapers or curves. While the term tube may connote a cylindrical component, the inner tube 32 and/or the passage 38 can have almost any cross-sectional shape. Preferably, the inner tube 32 and/or the passage 38 are substantially circular in cross-section. The inner tube 32 and/or the passage 38 can also be, for example, substantially oval, rectangular, triangular or polygonal in cross-section. Whatever the geometry, the cross-sectional area of the inner tube 32 and/or the passage 38 is preferably substantially constant. The inner tube 32 and/or the passage 38 can be sized for the particular application at hand. The inner tube 32 can be made of any suitable material.

The inner tube 32 can have an inlet end 40 and a termination end 42. Each of the inlet end 40 and the termination end 42 can be substantially open. The inlet end 40 can be adapted for attachment to a pressurized chamber, such as the combustor section 14 of a turbine engine 10. Once attached, the passage 38 and the pressurized chamber can be in fluid communication. The attachment between the inlet end 40 of the inner tube 32 and the pressurized chamber can be achieved in numerous ways including, for example, by fasteners, adhesives, welding, and mechanical engagement. In any event, a pressure signal 44 from the pressurized chamber can be received within the passage 38.

A pressure measuring device, such as a pressure transducer 50, can be operatively connected in branched relation to the passage 38. The pressure measuring device can be located along the passage 38 between the inlet end 40 and the termination end 42, preferably closer to the termination end 42. In one embodiment, pressure measuring device can be oriented at about 90 degrees relative to the passage 38, but other relative orientations are possible. An opening (not shown) can be provided in the inner tube 32 to receive a portion of the pressure measuring device. The pressure measuring device can be connected to the inner tube 32 in various ways including by fasteners, welding, adhesives, and mechanical engagement.

The system can include an outer tube 54. The outer tube 54 have an outer peripheral surface 56 and an inner peripheral surface 58. The outer tube 54 can have a substantially hollow interior 59. The outer tube 54 can have an at least partially open first end 60 and a closed second end 62. As noted earlier, the term “tube” may connote a cylindrical shape, but the outer tube 54 can have almost any cross-sectional shape including, for example, circular, oval, rectangular, polygonal, and triangular. In one embodiment, the shape of the outer peripheral surface 34 of the inner tube 32 can substantially correspond to the shape of the inner peripheral surface 58 of the outer tube 54. The outer tube 54 can be made of any suitable material and is preferably made of the same material as the inner tube 32.

The inner tube 32 can operatively engage the first end 60 of the outer tube 54 such that the passage 38 is in fluid communication with the interior 59 of the outer tube 54. Thus, the interior 59 can receive pressure signal 44 exiting the passage 38. There are various ways in which the inner tube 32 and the outer tube case be operatively engaged. For example, a portion of the inner tube 32 including the termination end 42 can extend into the interior 59 of the outer tube 54, as shown in FIG. 4A. However, the entire inner tube 32 does not extend into the outer tube 54. The inner tube 32 can be positioned substantially coaxially within the outer tube 54. A space 64 can be defined between the outer peripheral surface 34 of the inner tube 32 and the inner peripheral surface 58 of the outer tube 54, and between the termination end 42 of the inner tube 32 and the closed second end 62 of the outer tube 54.

A substantial portion of the inner tube 32 is free of apertures; however, a portion of the inner tube 32 can include a plurality of apertures 52. The location of these apertures 52 on the inner tube 32 will be described in detail below. The apertures 52 can be any size or shape and can extend at any orientation through the inner tube 32 from the inner peripheral surface 36 to the outer peripheral surface 34. In one embodiment, the apertures 52 can be substantially identical in size and shape. However, one or more of the apertures 52 can be a different size and/or shape from the other apertures 52. The apertures 52 can be substantially circular, but other shapes are possible including oval, rectangular, triangular, polygonal or, just to name a few possibilities. The apertures 52 can be arranged according to a pattern or to no pattern at all. In one embodiment, the apertures 52 can be equally spaced from each other.

It will be noted that the portion of the inner tube 32 that includes apertures 52 is disposed entirely within the outer tube 54. In other words, the portion of the inner tube 32 containing apertures 52 does not extend substantially beyond a plane defined by the first end 60 of the outer tube 54.

As noted above, the first end 60 of the outer tube 54 can be at least partially open. It is preferred if the first end 60 of the outer tube 54 is open just enough to allow passage of the inner tube 32 so a close fit is achieved. In one embodiment, shown in FIG. 4, a separate end piece 66 can be attached to the outer tube 54 to form the first end 60. The end piece 66 can be attached to the outer tube 54 in any of a number of ways, such as by threaded engagement 68 or/and fasteners (not shown). Thus, the end piece 66 can be selectively attached to and removed from the outer tube 54 as needed. The end piece 66 should remain attached during engine operation. The end piece 66 can include an opening 70 to allow the inner tube 32 to pass therethrough. Any space between the first end 60 of the outer tube 54 and the outer peripheral surface 34 of the inner tube 32 can be closed by, for example, a seal.

In another embodiment, as shown in FIG. 4B, the termination end 42 of the inner tube 32 can engage the first end 60 of the outer tube 54. For instance, the termination end 42 of the inner tube 32 can be positioned substantially proximate the first end 60 of the outer tube 54. The inner tube 32 may or may not be attached to the first end 60 of the outer tube 54.

A substantial portion of the interior 59 of the outer tube 54 can be filled with sound absorption material or dampening material 72. In the case of the embodiment shown in FIG. 4A, at least a portion of the space 64 between the inner tube 32 and the outer tube 54 can be filled with a sound absorption or dampening material 72, thereby forming an acoustic termination 74. Preferably, substantially the entire space 64 is filled with the sound absorption material 72. Thus, the sound absorption material 72 can surround the outer peripheral surface 34 of the inner tube 32 as well as the termination end 42 of the inner tube 32. The sound absorption material 72 is only needed in the space 64; it is not needed around the portion of the inner tube 32 that extends outside of the outer tube 54, which can provide appreciable cost savings.

In the case of the embodiment shown in FIG. 4B, it is preferred if the sound absorption material 72 is provided in the outer tube 54 so as to define a chamber 75. The chamber 75 can be in fluid communication with the passage 38 to receive pressure signals 44 therefrom. The chamber 75 can be almost any shape or size. Preferably, the chamber 75 is sized and/or shaped to substantially maintain acoustic continuity so as to minimize reflection.

The sound absorption material 72 can be any material that can dissipate acoustic energy in the pressure signal 44 in the passage. In one embodiment, the sound absorption material 72 can be fiber glass or polymer foam. According to one study, a typical fiber glass with a thickness of about 4 inches (that is, the distance between the outer peripheral surface 34 of the inner tube 32 and the inner peripheral surface 58 of the outer tube 54 and/or the distance between the termination end 42 of the inner tube 32 and the second end 62 of the outer tube 54) can dissipate about 60% of the acoustic energy in the pressure signal 44 for low frequencies, up to 100 Hertz. As the thickness of the fiber glass is increased, the amount of dissipation can increase, approaching from about 95% to about 100%. Therefore, by adjusting the thickness of the sound absorption material 72, the amount of dissipation can be adjusted to achieved the desired performance of the acoustic termination 74.

In one embodiment, the sound absorption material 72 can be provided as a pre-treated or pre-formed component that is simply placed inside the outer tube 54. The pre-formed sound absorption material 72 can provide an opening sized for receiving the extending portion of the inner tube 32. Alternatively, the termination end 42 of the inner tube 32 can be inserted in the outer tube 54 and the sound absorption material 72 can be packed in the space 64 therebetween.

Aside from the thickness of the sound absorption material 72 discussed above, it will be appreciated that other features of the system 30 can be optimized to achieve the desired performance of the acoustic termination 74. Examples of such features include but are not limited to the following: the length of the outer tube 54; the length of the inner tube 32; the cross-sectional area of the inner peripheral surface 58 of the outer tube 54; the cross-sectional area of the inner and outer peripheral surfaces 34, 36 of the inner tube 32; the length which the inner tube 32 extends into the outer tube 54; and the size, shape and position of the apertures 52 in the inner tube 32.

The operation of the pressure monitoring system 30 according to aspects of the invention will now be described. A pressure signal 44 from the combustor section 14 can enter the passage 38 at the inlet end 40. The pressure signal 44 can be in the form of a wave. The pressure signal 44 can travel along the passage 38 toward the termination end 42. Because the inner tube 32 does not have apertures 52 between the inlet end 40 and the acoustic termination 74, there is minimal, if any, loss of the pressure signal 44 as it travels along the passage 38. If apertures 52 were provided along the inner tube 32 upstream of the acoustic termination 74, losses in the pressure signal 44 would occur as the pressure signal 44 traveled through the passage 38, which can potentially result in a inaccurate pressure measurement.

The pressure signal 44 can engage the pressure measuring device, which can use the signal to measure or otherwise determine the pressure of the pressurized chamber. The pressure signal 44 then travels to the acoustic termination 74.

As noted earlier in connection with the embodiment shown in FIG. 4A, the portion of the inner tube 32 extending into the outer tube 54 includes a plurality of apertures 52. Thus, some of the pressure signal 44 can exit the inner tube 32 through the apertures 52. The energy of these exiting signals can be dissipated by the surrounding sound absorption material 72. The remainder of the pressure signal 44 entering the acoustic termination 74 can exit the inner tube 32 through the open termination end 42. The acoustic energy of these exiting signals can likewise be dissipated by the sound absorption material 72.

As for the embodiment of the acoustic termination 74 shown in FIG. 4B, the pressure signal 44 can exit the inner tube 32 through the termination end 42. Such exiting signals can be received in the chamber 75 and dissipated by the surrounding sound absorption material 72.

Thus, it will be appreciated that the acoustic termination 74 according to aspects of the invention can substantially dissipate the pressure signal 44 and minimize reflection of the pressure signal 44 into the passage 38. As a result, distortion or contamination of the pressure signal 44 in the passage 38 can be minimized.

The acoustic termination 74 according to aspects of the invention can result in an appreciable improvement in performance of the pressure monitoring system 30. Based on one model, the signal distortion associated with the prior pressure monitoring system 18 described above is about 10 to 20 percent; that is, about 10 to about 20 percent of the pressure signal 44 is reflected off of the termination 22. According to the same model, the signal distortion associated with the system 30 according to aspects of the invention can be about 5 to about 10 percent. In other words, about 5 to about 10 percent of the pressure signal 44 is reflected off of the acoustic termination 74 according to aspects of the invention. Thus, the acoustic termination 74 according to aspects of the invention can reduce signal distortion overall by up to about 50 percent to about 75 percent. In addition to improving system performance, the acoustic termination 74 of the pressure monitoring system 30 according to aspects of the invention can be made relatively easier and at a lower cost compared to the prior acoustic termination 22.

The foregoing description is provided in the context of one possible pressure monitoring system 30. While described in the context of turbine engines, it will be appreciated that aspects of the invention are not limited to turbine engines and can be used in connection with a variety of applications. Thus, it will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention as defined in the following claims. 

1. An acoustic termination for a pressure monitoring system comprising: an outer tube having an at least partially open first end and a closed second end, the outer tube having an interior, wherein at least a portion of the interior is filled with sound absorption material; and an inner tube having an open inlet end and an open termination end, the inner tube further having an outer peripheral surface and an inner peripheral surface, the inner peripheral surface defining a passage extending through the inner tube, the inner tube operatively engaging the first end of the outer tube such that the passage is in fluid communication with the interior of the outer tube; whereby a pressure signal transmitted through the inner tube is received within the interior of the outer tube and dampened by engagement with the sound absorption material.
 2. The acoustic termination of claim 1 wherein the sound absorption material is fiberglass.
 3. The acoustic termination of claim 1 wherein the first end of the outer tube provides an opening sized to allow passage of the inner tube such that the opening substantially engages the outer peripheral surface of the inner tube.
 4. The acoustic termination of claim 1 wherein the inlet end of the inner tube is connected to a pressurized chamber.
 5. The acoustic termination of claim 4 wherein the pressurized chamber is the combustor of a gas turbine engine.
 6. The acoustic termination of claim 1 further including: a pressure measurement device operatively connected in branched relation to the passage, whereby the pressure measurement device determines pressure when it is engaged by a pressure signal in the passage.
 7. The acoustic termination of claim 6 wherein the pressure measurement device is a pressure transducer.
 8. The system of claim 1 wherein the termination end of the inner tube is positioned substantially proximate the first end of the outer tube, wherein a chamber is defined within the sound absorption material such that the passage of the inner tube is in fluid communication with the chamber, whereby a pressure signal exiting the inner tube is received within the chamber.
 9. The system of claim 1 wherein a portion of the inner tube including the termination end extends into the first end of the outer tube, wherein the inner tube includes a plurality of apertures only in the portion of the inner tube that extends into the outer tube, and wherein the sound absorption material is provided in the space between the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube and the termination end of the inner tube and the second end of the outer tube.
 10. A pressure monitoring system comprising: a pressurized chamber containing at least one pressure signal; an outer tube having an at least partially open first end and a closed second end, the outer tube having an interior, wherein at least a portion of the interior is filled with sound absorption material; and an inner tube having an open inlet end and an open termination end, the inner tube further having an outer peripheral surface and an inner peripheral surface, the inner peripheral surface defining a passage extending through the inner tube, the inner tube operatively engaging the first end of the outer tube so as to be in fluid communication with the interior of the outer tube; whereby a pressure signal transmitted through the inner tube is received within the interior of the outer tube and dampened by engagement with the sound absorption material.
 11. The system of claim 10 wherein the pressurized chamber is the combustor of a gas turbine engine.
 12. The system of claim 10 wherein the sound absorption material is fiberglass.
 13. The system of claim 10 wherein the termination end of the inner tube is positioned substantially proximate the second end of the outer tube, wherein a chamber is defined within the sound absorption material such that the passage of the inner tube is in fluid communication with the chamber, whereby a pressure signal exiting the inner tube is received within the chamber.
 14. The system of claim 10 wherein a portion of the inner tube including the termination end extends into the first end of the outer tube, wherein the inner tube includes a plurality of apertures only in the portion of the inner tube that extends into the outer tube, and wherein the sound absorption material is-provided in the space between the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube and the termination end of the inner tube and the second end of the outer tube.
 15. The acoustic termination of claim 10 further including: a pressure measurement device operatively connected in branched relation to the passage, whereby the pressure measurement device determines pressure when it is engaged by a pressure signal in the passage.
 16. The acoustic termination of claim 15 wherein the pressure measurement device is a pressure transducer.
 17. A pressure monitoring system comprising: an outer tube having an at least partially open first end and a closed second end, the outer tube having an inner peripheral surface; an inner tube having an open inlet end and an open termination end, the inner tube further having an outer peripheral surface and an inner peripheral surface, the inner peripheral surface defining a passage extending through the inner tube, wherein a portion of the inner tube including the termination end extends into the first end of the outer tube, wherein the inner tube includes a plurality of apertures only in the portion of the inner tube that extends into the outer tube; a pressure measurement device operatively connected in branched relation to the passage; and sound absorption material provided in the space between the outer peripheral surface of the inner tube and the inner peripheral surface of the outer tube and the termination end of the inner tube and the second end of the outer tube, whereby a pressure signal transmitted through the inner tube engages the pressure measurement device so that the device determines pressure, and whereby the pressure signal is substantially dampened by engagement with the sound absorption material after exiting the inner tube through the apertures and the termination end.
 18. The system of claim 17 wherein the inlet end of the inner tube is connected to the combustor of a gas turbine engine.
 19. The system of claim 17 wherein the sound absorption material is fiberglass.
 20. The system of claim 17 wherein the pressure measurement device is a pressure transducer. 